Dirty fuel filter
Diesel engines do not engine brake like gasoline engines do, so relatively speaking, the rate of deceleration would seem to be slower.
There is no such engine as a Cummins 2500 Diesel.
pree ignition timing is advanced
one of the causes is bad low quality fuel
There are several causes for high fuel consumption in a diesel engine. These include a difference in the weight of the vehicle, increased usage of air conditioning, and a difference in tire pressure.
unburnt fuel
Blue smoke is burning oil.
You eating the engine.
dirty fuel filters for one !!
The diesel engine doesn't have any spark plugs, and uses heat generated during compression to ignite the fuel. The diesel however, is an internal combustion engine.
Just going on the information you've provided in the question, it's doubtful that the engine light is the cause of the deceleration. It's more likely that the light is correctly indicating the presence of whatever malfunction in the engine is causing the deceleration, and the engine malfunction is what you need to identify. Your local friendly mechanic is your best choice.
Take a look at to oxygen sensor. A dead sensor can cause your engine to surge and miss. If you live in the north were the fuel is oxygenated it happens more often. Something about O2 sensors being killed by oxygenated fuel.